Internet Engineering Task Force T. Narten Internet-Draft IBM Intended status: Informational October 18, 2011 Expires: April 20, 2012 Problem Statement for ARMD draft-ietf-armd-problem-statement-00 Abstract This document examines issues related to the massive scaling of data centers. Our initial scope is relatively narrow. Specifically, we focus on address resolution (ARP and ND) within the data center. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on April 20, 2012. Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Narten Expires April 20, 2012 [Page 1] Internet-Draft armd-problem October 2011 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Representative Data Center Designs . . . . . . . . . . . . . . 6 4.1. Scenario 1: L3 Terminates at the Access Link . . . . . . . 6 4.2. Scenario 2: L3 Terminates at the Aggregation Switch . . . 7 5. Address Resolution in IPv4 . . . . . . . . . . . . . . . . . . 7 6. Problem Itemization . . . . . . . . . . . . . . . . . . . . . 8 6.1. ARP Processing on Routers . . . . . . . . . . . . . . . . 8 6.2. MAC Address Table Size Limitations in Switches . . . . . . 9 7. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 8. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 10 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 11. Security Considerations . . . . . . . . . . . . . . . . . . . 10 12. Informative References . . . . . . . . . . . . . . . . . . . . 10 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10 Narten Expires April 20, 2012 [Page 2] Internet-Draft armd-problem October 2011 1. Introduction This document examines issues related to the massive scaling of data centers. Specifically, we focus on address resolution (ARP in IPv4 and Neighbor Discovery in IPv6) within the data center. Although strictly speaking the scope of address resolution is confined to a single L2 broadcast domain (i.e., ARP runs at the L2 layer below IP), the issue is complicated by routers with many interfaces (on which address resolution is performed) or with IEEE 802.1Q domains, where individual VLANs form their own broadcast domains. Thus, the scope of address resolution spans both the L2 link and the devices attached to those links. This document is intended to support the ARMD WG identify potential future work areas. The scope of this document intentionally starts out relatively narrow, mirroring the ARMD WG charter. Expanding the scope requires careful thought, as the topic of scaling data centers generally has an almost unbounded potential scope. This document aims to list "pain points" that are being experienced in current data centers. It is separate exercise to determine which (if any) of these pain points should lead to specific protocol work, whether in ARMD or some other WG. 2. Terminology Application: a service that runs on either a physical or virtual machine, providing a service (e.g., web server, database server, etc.) Broadcast Domain: The set of all links and switches that are traversed in order to reach all nodes that are members of a given L2 domain. For example, when sending a broadcast packet on a VLAN, the domain would include all the links and switches that the packet traverses when broadcast traffic is sent. Host (or server): Physical machine on which a system is run. A system can consist of an application running on an operating system on the "bare metal" or multiple applications running within individual VMs on top of a hypervisor. Traditional non- virtualized systems will have a single (or small number of) IP addresses assigned to them. In contrast, a virtualized system will use many IP addresses, one for the hypervisor plus one (or more) for each individual VM. Narten Expires April 20, 2012 [Page 3] Internet-Draft armd-problem October 2011 Hypervisor: Software running on a host that allows multiple VMs to run on the same host. L2 domain: IEEE802.1Q domain supporting up to 4095 VLANs. The notion of an L2 broadcast domain is closely tied to individual VLANs. Broadcast traffic (or flooding to reach all destinations) reaches every member of the specific VLAN being used. Virtual machine (VM): A software implementation of a physical machine that runs programs as if they were executing on a bare machine. Applications do not know they are running on a VM as opposed to running on a "bare" host or server. 3. Background Large, flat L2 networks have long been known to have scaling problems. As the size of an L2 network increases, the level of broadcast traffic from protocols like ARP increases. Large amounts of broadcast traffic pose a particular burden because every device (switch, host and router) must process and possibly act on such traffic. In addition, large L2 networks can be subject to "broadcast storms". The conventional wisdom for addressing such problems has been to say "don't do that". That is, split large L2 networks into multiple smaller L2 networks, each operating as its own L3/IP subnet. Numerous data center networks have been designed with this principle, e.g., with each rack placed within its own L3 IP subnet. By doing so, the broadcast domain (and address resolution) is confined to one Top of Rack switch, which works well from a scaling perspective. Unfortunately, this conflicts in some ways with the current trend towards dynamic work load shifting in data centers and increased virtualization as discussed below. Workload placement has become an issue within data centers. Ideally, it is desirable to be able to move workloads around within a data center in order to optimize server utilization, add additional servers in response to increased demand, etc. However, servers are often pre-configured to run with a given set of IP addresses. Placement of such servers is then subject to constraints of the IP addressing restrictions of the data center. For example, servers configured with addresses from a particular subnet could only be placed where they connect to the IP subnet corresponding to their IP addresses. If each top of rack switch is placed within its own subnet, a server can only be connected to the one top of rack switch. This same constraint occurs in virtualized environments, as discussed next. Server virtualization is fast becoming the norm in data centers. Narten Expires April 20, 2012 [Page 4] Internet-Draft armd-problem October 2011 With server virtualization, each physical server supports multiple virtual servers, each running its own operating system, middleware and applications. Virtualization is a key enabler of workload agility, i.e., allowing any server to host any application and providing the flexibility of adding, shrinking, or moving services among the physical infrastructure. Server virtualization provides numerous benefits, including higher utilization, increased data security, reduced user downtime, and even significant power conservation, along with the promise of a more flexible and dynamic computing environment. The discussion below focuses on VM placement and migration. Keep in mind, however, that even in a non-virtualized environment, many of the same issues apply to individual workloads running on standalone machines. For example, when increasing the number of servers running a particular workload to meet demand, placement of those workload may be constrained by IP subnet numbering considerations. The greatest flexibility in VM and workload management occurs when it is possible to place a VM (or workload) anywhere in the data center regardless of what IP addresses the VM uses and how the physical network is laid out. In practice, movement of VMs within a data center is easiest when VM placement and movement does not conflict with the IP subnet boundaries of the data center's network, so that the VM's IP address need not be changed to reflect its actual point of attachment on the network from an L3/IP perspective. In contrast, if a VM moves to a new IP subnet, its address must change, and clients will need to be made aware of that change. From a VM management perspective, management is simplified if all servers are on a single large L2 network. With virtualization, a single physical server can host 10 (or more) VMs, each having its own IP (and MAC) addresses. Consequently, the number of addresses per machine (and hence per subnet) is increasing, even when the number of physical machines stays constant. Today, it is not uncommon to support 10 VMs per physical server. In a few years, the number will likely reach 100 VMs per physical server. In the past, services were static in the sense that they tended to stay in one physical place. A service installed on a machine would stay on that machine because the cost of moving a service elsewhere was generally high. Moreover, services would tend to be placed in such a way as to facilitate communication locality. That is, servers would be physically located near the services they accessed most heavily. The network traffic patterns in such environments could thus be optimized, in some cases keeping significant traffic local to one network segment. In these more static and carefully managed environments, it was possible to build networks that approached Narten Expires April 20, 2012 [Page 5] Internet-Draft armd-problem October 2011 scaling limitations, but did not actually cross the threshold. Today, with the proliferation of VMs, traffic patterns are becoming more diverse and less predictable. In particular, there can easily be less locality of network traffic as services are moved for such reasons as reducing overall power usage (by consolidating VMs and powering off idle machine) or to move a virtual service to a physical server with more capacity or a lower load. In today's changing environments, it is becoming more difficult to engineer networks as traffic patterns continually shift as VMs move around. In summary, both the size and density of L2 networks is increasing. In addition, increasingly dynamic workloads and the increased usage of VMs is creating pressure for ever larger L2 networks. Today, there are already data centers with 120,000 physical machines. That number will only increase going forward. In addition, traffic patterns within a data center are changing. 4. Representative Data Center Designs This section outlines some general data center designs and how they impact address resolution. These designs may only approximate what happens in real data centers, but it is hoped that they can serve as a useful vehicle for describing pain points that are being experienced today in current data centers. Many data centers build their L2 networks using a two-tier approach consisting of access and aggregation switches. Servers connect to access switches (e.g., top-of-rack switches) and access switches in turn are interconnected via aggregation switches. In the following, we describe two common layouts. 4.1. Scenario 1: L3 Terminates at the Access Link In Scenario 1, the L3 network extends all the way to the access switches, with the L2 broadcast domain terminated at the access switch. All servers attached to an access switch are part of the same L2 broadcast domain and the same IP subnet. Each access switch terminates its own L2 broadcast domain, and machines connected to different access switches are numbered out of different IP subnets. This approach works well from an address resolution perspective because the overall number of machines (physical and virtual) in a single L2 domain is relatively small, e.g., in the low hundreds. The main disadvantage to this scenario is that VMs cannot easily be moved from a server attached to one access switch to a server on a different access switch, as doing so requires changing the VM's IP Narten Expires April 20, 2012 [Page 6] Internet-Draft armd-problem October 2011 address, or taking additional steps at the IP routing level to ensure that traffic continues to reach the VM at its new location, even though its IP address no longer matches the subnet configuration of the physical network. 4.2. Scenario 2: L3 Terminates at the Aggregation Switch In Scenario 2, the L3 network extends only to the aggregation switches (or perhaps to routers that connect to the aggregation switches). The aggregation switches (or the routers that connect to multiple aggregation switches) could terminate multiple distinct IP subnets (e.g., one per VLAN) or one large IP subnet. In order to let hosts belonging to different IP subnets be placed under any access switches, it is necessary for access switches to enable multiple VLANs and aggregation switches to enable some VLANs (or subnets) over many physical ports. This configuration breaks the confinement of the VLAN's broadcast domain and makes it equivalent to all the access switches being part of the same L2 broadcast domain (and IP subnet). Thus, this configuration allows VMs to be moved to servers connected to other access switches, but increases the size of the L2 broadcast domain, which can lead to difficulties outlined below. 5. Address Resolution in IPv4 In IPv4, ARP provides the function of address resolution. To determine the link-layer address of a given IP address, a node broadcasts an ARP Request. The request is delivered to all portions of the L2 network, and the node with the requested IP address replies with an ARP response. ARP is an old protocol, and by current standards, is sparsely documented. For example, there are no clear requirement for retransmitting ARP requests in the absence of replies. Consequently, implementations vary in the details of what they actually implement [RFC0826][RFC1122]. From a scaling perspective, there are a number of problems with ARP. First, it uses broadcast, and any network with a large number of attached hosts will see a correspondingly large amount of broadcast ARP traffic. The second problem is that it is not feasible to change host implementations of ARP - current implementations are too widely entrenched, and any changes to host implementations of ARP would take years to become sufficient deployed to matter. That said, it may be possible to change ARP implementations in hypervisors, L2/L3 boundary routers, and/or ToR access switches, to leverage such techniques as Proxy ARP and/or OpenFlow infused directory assistance approaches. Finally, ARP needs to take steps in order to flush out stale or changed entries. However, the existing standards do not provide clear implementation guidelines for how to do this. Consequently, Narten Expires April 20, 2012 [Page 7] Internet-Draft armd-problem October 2011 some implementations are "chatty" in that they just periodically flush caches every few minutes and rerun ARP. 6. Problem Itemization This section articulates some specific problems or "pain points" that are related to large data centers. It is a future activity to determine which of these areas can or will be addressed by ARMD or some other IETF WG. 6.1. ARP Processing on Routers One pain point with large L2 broadcast domains is that the routers connected to the L2 domain need to process "a lot of" ARP traffic. Even though the vast majority of ARP traffic may well not be for that router, the router still has to process enough of the ARP request to determine it can safely be ignored. The ARP algorithm specifies that a recipient must update its ARP cache if it receives an ARP query from a source for which it has an entry [RFC0826]. A common router architecture has ARP processing handled in a "slow path" software processor rather than directly by a hardware ASIC as is the case when forwarding packets. Such a design significantly limits the rate at which ARP traffic can be processed. Current implementations today can support in the low thousands of ARP packets per second. To further reduce the ARP load, some routers have implemented additional optimizations in their ASIC fast paths. For example, some routers can be configured to discard ARP requests for target addresses other than those assigned to the router. That way, the router's software processor only recieves ARP requests for addresses it owns and must respond to. This can significantly reduce the number of ARP requests that must be processed by the router. Another optimization concerns reducing the number of ARP queries targeted at routers, whether for address resolution or to validate existing cache entries. Some routers can be configured to send out periodic gratuitous ARPs, helping to reduce the number of ARP queries they receive. The gratuitous ARP pre-populates the ARP caches on neighboring devices, or refreshes the "last validated" timestamp on such entries, reducing the number of ARP queries they send to the router. Finally, another area concerns how routers process IP packets for which no ARP entry exists. Such packets must be held in a queue while address resolution is performed. Once an ARP query has been Narten Expires April 20, 2012 [Page 8] Internet-Draft armd-problem October 2011 resolved, the packet is forwarded on. Again, the processing of such packets is handled in the "slow path". This effectively limits the number of ARP "cache misses" that a router can process and is viewed as a problem in some networks today. Although address-resolution traffic remains local to one L2 network, some data center designs terminate L2 subnets at individual aggregation routers (i.e., Scenario 2). Such routers can be connected to a large number of interfaces (e.g., 100). While the address resolution traffic on any one interface may be manageable, the aggregate address resolution traffic across all interfaces can become problematic. Another variant of Scenario 2 has individual routers servicing a relatively small number of interfaces, with the individual interfaces themselves serving very large subnets. Once again, it is the aggregate quantity of ARP traffic seen across all of the router's interfaces that can be problematic. This "pain point" is essentially the same as the one discussed above, the only difference being whether a given number of hosts are spread across a few large subnets or many smaller ones. 6.2. MAC Address Table Size Limitations in Switches L2 switches maintain L2 MAC address forwarding tables for all sources and destinations traversing through the switch. These tables are populated through learning and are used to forward L2 frames to their correct destination. The larger the L2 domain, the larger the tables have to be. While in theory a switch only needs to keep track of addresses it is actively using, switches flood broadcast frames (e.g., from ARP), multicast frames (e.g., from Neighbor Discovery) and unicast frames to unknown destinations. Switches add entries for the source addresses of such flooded frames to their forwarding tables. Consequently, MAC address table size can become a problem as the size of the L2 domain increases. The table size problem is made worse with VMs, where a single physical machine now hosts ten (or more) VMs, since each has its own MAC address that is visible to switches. In Scenario 1, the size of MAC address tables in switches s not generally a problem. In Scenario 2, however MAC table size limitations can be a real issue. [xxx: do we have numbers? For what size L2 broadcast domains do we start seeing problems? ] 7. Summary This document has outlined a number of problems or "pain points" Narten Expires April 20, 2012 [Page 9] Internet-Draft armd-problem October 2011 related to address resolution in large data centers. 8. Open Issues 1. The document concentrates on ARP, but the same analysis needs to be performed for IPv6's Neighbor Discovery. 9. Acknowledgments This document has been significanlty improved by comments from Linda Dunbar and Sue Hares. Igor Gashinsky deserves addition credit for highlighting some of the ARP-related pain points and for clarifying the difference between what the standards require and what some router vendors have actually implemented in response to operator requests. 10. IANA Considerations This document makes not request of IANA. 11. Security Considerations This documents lists existing problems or pain points with address resolution in data centers. This document does not create any security implications nor does it have any security implications. The security vulnerabilities in ARP are well known and this document does not change or mitigate them in any way. 12. Informative References [RFC0826] Plummer, D., "Ethernet Address Resolution Protocol: Or converting network protocol addresses to 48.bit Ethernet address for transmission on Ethernet hardware", STD 37, RFC 826, November 1982. [RFC1122] Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989. Narten Expires April 20, 2012 [Page 10] Internet-Draft armd-problem October 2011 Author's Address Thomas Narten IBM Email: narten@us.ibm.com Narten Expires April 20, 2012 [Page 11]